Off Center Fed (OCF) Dipole Antennas

(Often referred to as "WINDOM")

The two quotes below describe the problem of the OCFD in a nutshell.

These are the MAIN problems facing the OCFD!

The most difficult subjects can be explained to the most  slow-

witted man if he has not formed any idea of them already;  but

the simplest thing cannot be made clear to the most intelligent man if he is firmly persuaded that he knows already, without a shadow of a doubt, what is laid before him...Leo Telstoy, 1897.

Indeed, the biggest challenge we have in understanding these antennas is disregarding what we thought we already knew about them.

Mark Twain said basically the same thing but in simpler terms:

Indeed, the biggest challenge we face in understanding these antennas is disregarding what we thought we already knew  about them, and starting over with a clean sheet of paper!

 Although "OCFD" is the more accurate term for these antennas,

I like the name "WINDOM" much better

and sometimes use it interchangably in my writings on OCFD antennas . . .

(even though it is technically wrong.)

.

Whereas a Dipole is a half-wavelength long radiator, fed in the Center of two equal length legs of the antenna, the Off-Center-Fed Dipole (which I sometimes 'falsely' call a WINDOM) is a Dipole whose feedpoint is at a point removed from the center of the antenna - but not at its end.

The picture above on the left shows the classical Dipole, which is usually fed with 50 Ohm coax, through a 1:1 balun.

The picture on the right above shows an OCF Dipole with the feedpoint positioned at the same feedpint of the classical WINDOM antenna, fed with 50 Ohm coax through a 4:1 or 6:1 balun at a point 1/3 of the distance from one end.

Note: It is not imperative to feed the OCF antenna at the point shown.  Indeed, it is usually advantageous to choose a different position to feed it.  Changing the location of the feedpoint will  SIGNIFICANTLY  change the characteristics of this antenna.  In addition, a 4:1 balun is almost always a better choice than a 6:1 balun.

ANSWER:  to be able to work more bands with one antenna.

A dipole is basically a mono-band antenna.  It is sometimes used on its 3rd harmonic (i.e., a 40m dipole may also be used on 15m), but if it is fed with coax, it should not be used on its even harmonic frequencies.  That's because the impedance of the dipole on its even harmonics is too high (typically >2K Ohms) and feedline losses and losses in the balun are excessively high.

By moving the feedpoint away from the center of the dipole,

we enable the antenna to be used on more harmonic frequencies.  

We convert the mono-band antenna into a good multi-band antenna.

THE COST:  You will need to use a VERY GOOD 4:1 current balun.

CAUTION:  THIS IS NOT AN "ALL-BAND" ANTENNA, IT IS A "MULTI-BAND" ANTENNA.

- - - - - - - - - - 

In the past, I was a big fan of openwire fed antennas.  Indeed, I used these for nearly 50 years, any time I wanted a single wire antenna to cover multiple bands.  

I switched my multi-band antenna preference in 2011 from

openwire-fed/center-fed dipoles  to coax-fed OCF dipoles.

NOTE:   I SHALL USE THE FOLLOWING TERMINOLOGY BELOW:

• OCFD:      Off Center Fed Dipole

• OWCFD:  Open Wire Center Fed Dipole

Some will argue that the 'openwire-fed/center-fed' dipole is the most efficient antenna there is, when you are looking to cover 5, 6, 7, or even more bands with just one wire antenna.  

I don't dispute this point, although, if built right, the difference is insignificant on almost every band.  

AND, I do challenge the openwire-fed dipole's "perceived" postion of being the "best" solution no matter what.  I suggest we consider the following:

   Openwire-Fed Dipoles  REQUIRE  a Antenna Matchbox - on all bands, all the time.

AND that is their BIGGEST disadvantage.  But it doesn't stop there . . .

It requires a very good matchbox with high efficiency over a very broad impedance matching range. 

Some "claim" the best way to match an OWCFD is with a true "Balanced Matchbox".  These matchboxes are rare and very expensive compared to the more common asymmetrical matchboxes.   It has taken me many years to realize that this method is not the only way to efficiently achieve multi-band performance.  I have finally woken up!

In the meantime, several independent field tests of efficiency when matching openwire fed antennas have shown that, when done right, unbalanced (asymmetrical) antenna matchboxes can equal and in some cases even exceed the performance of the balanced matchboxes... in terms of efficiency, balance, and phase.

SEE:  ANTENNA MATCHBOX SHOOTOUTS
 

ALTERNATIVE:

INSTEAD of feeding the dipole in the middle with openwire, let's consider feeding it off-center with coax,  through a 4:1 balun.  It certainly works!  But what is the downside?

The downside is slightly higher losses, which when done right,

are negligable on most of the bands that the antenna covers.

HOWEVER, AS LONG AS THE FEEDLINE IS LESS THAN 100 FEET LONG, and fed with good quality coax,  the difference remains  LESS THAN 1 dB*  “if” you  build it right.  

*Reality Check:  Many people using openwire feedline today are not using the correct type of matchbox and/or, if using a populat T-Network tuner are not tuning it properly.  As a result, they  incurr  MORE LOSS IN THEIR MATCHBOX  than the loss incurred when using an OCFD instead of an OWCFD.

Done right, the performance and efficiency of the OCFD approaches that

of the 'openwire-fed/center-fed dipole', WITHOUT all the disadvantages.

So what can be done wrong with an OCFd?

The problem with OWCFD antennas like the "double-zepp" is, antennas resonant on their fundamental band have a ridiculously high impedance on their 2nd harmonic.  It can be difficult, sometimes even impossible to match the 2nd harmonic band without having to change the feedline length, unless you use Monster Matchboxes. 

BTW, "Monster Matchboxes"

capable of handling high power

are very   E-X-P-E-N-S-I-V-E !

Despite this fact, I owned and used Monster Matchboxes for many years and was completely satisfied . . .  but I spent a lot of money on matchboxes !!!

Now that I am retired, I am no longer able to throw bundles of cash towards matchbox purchases.  I have taken a more pragmatic look at the realities.  Today the OCFD is much more appealing to me than an openwire-fed dipole.

WHAT ABOUT PORTABLE OPERATIONS?

I have always been a great fan of portable operations, and when working portable, it is preferable to use antennas which require no matchbox at all, or if one is required, then it should suffice to use the radio's built in ATU, or something tiny and low-cost.

I found this to be impossible with the traditional  openwire-fed dipole.

I studied the impedance variance of dipoles as a relation of their feedpoint position and noticed that you don’t have to move too far away from a worst case scenario to achieve a much more match-friendly impedance on most of the bands.  I then sat about to find a combination of length and feedpoint position that enabled me to present a “civilized load” to the transceiver.  

"Civilized" meaning here that the antenna has close to 50 Ohms, or at least

that it can be matched by the radio's built-in ATU efficiently, or by a low-cost asymmetrical matchbox with external 1:1 balun.

I have used Ten-Tec transceivers for my home station for most of my 50 years on the air, because they have no problem to deliver full power, 100w all day long into a 3:1 SWR. 

Many transceivers don’t like this and fold back their output power in the presence of even moderate levels of SWR.  These tranceiver require a matchbox when others do not.  

PEOPLE SHOULD ALSO CONSIDER THIS  DOWNSIDE  

WHEN MAKING TRANSCEIVER PURCHASING DECISIONS.

I have used JA transceivers with built-in ATUs that could match at least 5:1.  Elecraft and Kenwood transceivers could match 10:1.   However, modern ICOM transceivers' built-in ATU can only match up to 3:1 SWR (with full power).

This, together with an OCFD has enabled me to work many bands without the need for an expensive symmetrical matchbox.  

BUT IT MUST BE AN OCFD WITH A GOOD DESIGN, NOT JUST ANY OCFD!

Many hours of computer modeling were followed by 10x as many hours in the field, measuring, and then evaluating the OCFD antenna results on my computer.  Nothing seemed to work like I wanted it to.

I was almost ready to give up, when I got the idea to replace my MFJ-259B analyzer with a RigExpert AA-54 analyzer.  The AA-54 can scan the entire HF spectrum within a few seconds, thus saving me loads of time spent taking multiple manual measurements across each band. 

After I became familiar with this new analyzer, I quickly was able to design my 7-band 80m OCFD and soon afterwards developed a 5-band 40m OCFD.  Voila!

If you are serious about building your own OCFD antenna, do

 yourself a favor and buy a good scanning antenna analyzer!

I now have two OCFD antennas in operation:

• originally a coax-fed 7-band 80m OCF for home use
  • ​now with a different feedpoint and balun:  807-HD
• originally a coax-fed 5-band 40m OCF for portable use
  • ​now with this design:  404-UL

I generally do not need to use an antenna matchbox on most bands;  

EXCEPTIONS:  

• 80m of course.  I don't need a matchbox in the CW portion of the band - only when working in the higher SSB portionof the band.  No dipole ever covers the entire 80m band without the use of a matchbox, unless you build something very exotic such as cage dipoles, etc.

• On one or two of the WARC bands, depending on which version of the antenna I am using.

CHOOSE NEXT PAGE from the menu on the LEFT, near the top of this page.

ADDITIONAL INFORMATION:

• OCFD:  https://www.dj0ip.com/ocfd 

• OWCFD:  [Open-Wire-Fed ANT]